Hypothermia- Exam 1 Flashcards
What is the hypothalamus responsible for?
Vascular regulation (vasoconstriction of skin vessels in cold) Skeletal Muscle activity (vasodilation of skeletal muscle vascular beds in cold)
What does the endocrine system do?
Metabolic regulation
Stress response
What system does hypothermia activate?
Endocrine system
Hypothalamus triggers ___________ nervous system response (triggered by the cold).
Sympathetic
Cold is sensed by what in the skin?
Thermoreceptors
What does activation of the endocrine system result in?
Oxygen consumption increases
HR increases
CO increases
BP increases
What temperature is mild hypothermia?
32-35 C
What temperature is moderate hypothermia?
28-31 C
What temperature is deep hypothermia?
18-27 C
What temperature is profound hypothermia?
< 18 C
Why induce hypothermia?
During bypass it provides degree of orange and organism protection; margin of safety
3 Mechanisms of Protection in hypothermia
- Reduction in metabolic rate and O2 Consumption
- Preservation of high-energy phosphate stores
- Reduction of excitatory neurotransmitters release due to ischemia
What happens with glutamate during ischemia?
It accumulates, opens calcium channels and activates multiple destructive enzymatic systems
Which is longer: safe period of hypothermia CPB or period predicted on the basis of reduced metabolic activity alone?
Safe period of hypothermia CPB
What are the general effects of hypothermia?
Allows lower pump flows
Better myocardial protection
less blood trauma (low flow = low damage)
Better overall organ protection
What is the cardiac index for 34-37 C?
2.4 L/min/m2
What is the cardiac index for 30-34 C?
2.0 L/min/m2
What is the cardiac index for 25-30 C?
1.8 L/min/m2
What is the cardiac index for 20-18 C?
1.5 L/min/m2
What is the cardiac index for 18 C?
1.0 L/min/m2
Methods of Hypothermia Induction
Surface cooling (ice) Surface cooling w. supplementary partial bypass Core cooling - Total ECC (DHCA, HLFB, HILFB)
How is surface cooling related to size?
Inverse effect related to size; small infants less than 5kg
When do you use DHCA?
Complex congenital heart defects
operations on the aortic arch
operations where maintenance of venous drainage is difficult
What should you do if the arrest period is longer than 60 minutes?
use intermittent arrest with brief periods of hypothermic flow
low-flow hypothermic perfusion is safer than total arrest
What are the basic global effects of hypothermia?
Decreased energy
More bleeding
diminished neuro transmission, receptor function, and protein activity
Q10 Principle
relates an increase or decrease in reaction rates to a change in 10 C
If Q10=2, a decrease in temperature 10C will result in 50% reduction in reaction rates
What is the Q10 value for most physiological rates?
2-3
Q10 Range (humans)
1.9-4.2 (mean=2) Organ/tissue variability temperature variability (2.23 at 27-30C) 4.5 at 14-27C
7 degree C Principle
Every 7C drop in temperature will result in a 50% decrease in oxygen consumption (and demand)
How do the levels of hypothermia relate to decrease in oxygen consumption?
Mild: 25% decrease
Moderate: 50 % decrease
Deep: 75% decrease
Profound: 87.5 % decrease
At 30C VO2 is what percent of normal?
50% normal
At 23C VO2 is what percent of normal?
25% normal
What does pH respond to?
Changes in CO2 content, not changes in the CO2 components
What does temperature change? (CO2 content or proportion of contents?)
Does not change O2 or CO2 content; just the proprotion of the components (dissolved vs partial pressure)
Gas solubility is ______________ related to temperature.
Inversely
Equation for gas content in solution
Gas content in solution = pp of gas x solubility
Henry’s Law
As pressure increases, solubility of gases in liquids increases
As temperature increases, solubility of gases in liquids decreases
Temperature is _________ related to partial pressure.
Directly
Temperature is __________ related to solubility.
Inversely
Increased temperature ->_________solubility–> _______ partial pressure
decreased, increased
decreased temperature–> __________ solubility –> _______ partial pressure
increased, decreased
Recommended rate for cooling?
1C per min
Recommended rate for warming?
1C per 3-5 minutes
Consequences on warming too fast
development of temp gradients within tissues
body cooling after bypass
exposure to hyperthermia
What are the limitations of rate of cooling/warming?
Water temp of heat exchanger
BP and SVR
Flow Rate
Which temperature gradients do we have to worry about?
Btw heat exchanger and venous blood
btw arterial blood and patient
Temperature gradients for adults and peds
Adults: < 10 C
Peds: < 8 C
Keep arterial pO2 below what?
<200mmHg
What could cause hyperthermia?
Time pressure of rewarming
Efficiency of heat exchanger (more or less than expected)
High water temperatures
Perfusionist inattention
DHCA Temperature
18-20C
How long do we turn off the pump in DHCA?
30-60 minutes
What organ is at greatest risk in DHCA?
overall brain function, developmental capacity & IQ
What is the safe arrest time for normothermia?
4-5 min
O2 consumption 100%
What is the safe arrest time for Moderate hypothermia?
8-10 min
O2 consumption 50%
What is the safe arrest time for deep hypothermia?
16-20 min
O2 consumption 25%
What is the safe arrest time for profound hypothermia?
32-40 min
o2 consumption 12%
DHCA Pro’s
Allows exposure
reduces metabolic rate and molecule movement
cessation of circulation
excitatory neurotransmitter reduction
DHCA Con’s
Neurologic Injury & morbidity
BRain is at the most risk
>60 min is detrimental
>40 min increases risk
How would you do a combined approach? (HLFB and HILFB)
DHCA with intermittent low flow bypass for 1-2 minutes every 15-20 minutes
How do you increase tolerance of brain to ischemic result?
Thiopental - short acting barbituate
Solumedrol- anti-inflammatory, stabilize cell membranes
What is a short acting baribituate to increase tolerance of brain to ischemic insult?
Thiopental
What is an anti-inflammatory that stabilizes cell membranes to incresa etolerance of brain to ischemic insult?
Solumedrol
